1. Field of the Invention
The invention relates to a transceiver, and more particularly, to a transceiver in-phase and quadrature (IQ) calibration system and associated method.
2. Description of the Related Art
A quadrature amplitude modulation (QAM) mode transceiver includes a transmitter and a receiver. The transmitter up-converts an in-phase (I) signal and a quadrature (Q) signal to a radio-frequency (RF) signal. The receiver receives the RF signal, and down-converts the RF signal to the I signal and the Q signal. Taking a 4QAM mode transmitter for example, the I signal and the Q signal are low-frequency signals having the same amplitude with a 90-degree phase difference.
The up-conversion from I and Q signals to an RF signal and the down-conversion from an RF signal to I and Q signals are performed by analog mixers. However, variables in the analog mixers lead to distortions and mismatches between the I and Q signals to further incur I/Q imbalance.
For example, an I channel and a Q channel in the transmitter respectively include a mixer, and the two mixers receive a pair of oscillation signals generated by a local oscillator (LO). When a phase difference between the pair of oscillation signals is not 90 degrees, a phase mismatch between the I and Q signals is incurred leading to a phase imbalance. Further, in the event that the I and Q channels adopt different circuit components, an amplitude mismatch between the I and Q signals is also incurred leading to a gain imbalance. Similarly, the receiver also suffers from the I/O imbalance due to the mismatch between the I and Q signals caused by the variables.
The amplitude imbalance in the RF signal of the transmitter degrades an error vector magnitude (EVM), and the phase imbalance misleads the receiver to treat the mismatched I and Q signals as correct signals. Therefore, to eliminate the overall I/Q imbalance, a calibration to the I and Q signals needs to be first performed in the transceiver.
In a conventional transceiver calibration method, either the transmitter or the receiver is first calibrated. For example, the relationship between the I and Q signals in the transmitter is first calibrated so that the mismatch between the I and Q signals in the transmitter is minimized. After calibrating the transmitter, the calibrated RF signal or an RF signal generated by an external tone generator is transmitted to the receiver to calibrate the receiver.
As noted, the above conventional calibration method involves an external signal source and other hardware devices. However, such components cannot be easily integrated with a system-on-chip (SoC).